This project is concerned with the physiological mechanisms which regulate fetal breathing movements. Human and animal (sheep, primates) fetuses in utero make breathing movements in periodic episodes which are present 30-40 percent of the time, in the last third of gestation. It has been suggested that the presence of normal fetal breathing movements may be an indication of fetal well-being prior to birth. We have shown that both carbon dioxide and metabolic acidosis are stimulants of fetal breathing movements in the sheep fetus. In sheep features we are now studying the role of cerebral spinal fluid acid-base balance in the regulation of fetal breathing movements. We have developed a preparation in unanesthetized fetal lambs in which central cerebral spinal fluid (CSF) can be perfused with artificial CSF. By varying the bicarbonate concentration in the CSF we can present varied hydrogen ion concentrations to the medullary respiratory chemoreceptors.